In a quantum communication system, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarization, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution (QKD) is a technique which results in the sharing of cryptographic keys between two parties: a transmitter often referred to as “Alice”; and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper, often referred to as “Eve”. In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.
It is desirable for quantum channels to co-exist with classical channels in a quantum communication system. In QKD techniques, Alice and Bob communicate using classical signals in addition to quantum signals. Classical and quantum channels may be transmitted together along a single optical fibre using wavelength division multiplexing, whereby different wavelengths of light are used to transmit the different signals.
When quantum and classical channels are multiplexed together in this way, Raman scattering of photons is generated by the high power classical lasers used to transmit the classical signals. This inelastic scattering of photons leads to noise in the quantum wavelength band which cannot be filtered with wavelength filters. In order to minimize this noise, the power of the classical signals may be restricted, for example.